PECVD is a process widely used in the manufacture of semiconductor devices for the deposition of layers of electronic materials on substrates including that of an insulating glass. In PECVD, a substrate body is placed in a vacuum deposition chamber equipped with a pair of parallel plate electrodes or other electrical sources. The substrate body is generally mounted on a susceptor which also serves as the lower electrode. A flow of a reactant gas is provided in the deposition chamber through a gas inlet manifold which also serves as the upper electrode. A radio frequency (RF) voltage is applied between the two electrodes which generate an RF power sufficient to cause a plasma to be formed in the reactant gas. The plasma causes the reactant gas to decompose and deposit a layer of the desired material on the surface of the substrate body. Additional layers of other electronic materials can be deposited on the first layer by providing in the deposition chamber a flow of a reactant gas containing the material of the additional layer to be deposited. Each reactant gas is subjected to a plasma which results in the deposition of a layer of the desired material.
In a conventional PECVD method used to produce silicon oxy-nitride films, an ammonia-based chemistry is used which requires a reactant gas mixture of silane, nitrous oxide, ammonia and nitrogen. In this ammonia-based reaction, silane supplies the silicon content of the film, nitrous oxide supplies the oxygen content of the film, ammonia supplies the nitrogen content of the film and nitrogen gas functions as a diluent. Ammonia is typically used as the source of nitrogen in a silicon oxy-nitride deposition process by the PECVD technique because it is very difficult to break down the triple bond structure in nitrogen gas.
In recent years, the deposition of silicon oxy-nitride films as a final passivation layer on a semiconductor device has become an important processing step in the manufacture of thin film transistors (TFT). This type of thin film transistor has been used to separately address areas of a liquid crystal cell contained between two glass plates at very fast rates. They are useful for active matrix displays such as those used in TV and computer monitors.
When a silicon oxy-nitride film is deposited as the final passivation layer on a thin film transistor, its deposition temperature is greatly limited due to the presence of the layers already built-up on the transistor and their sensitivity to high temperatures. The temperature limitation for the deposition process of the final silicon oxy-nitride layer is frequently limited to a temperature of less than 250.degree. C. This processing temperature is significantly lower than that normally used for the deposition of silicon oxy-nitride films on Si substrates by a PECVD process, i.e., between 350 to 450.degree. C.
At a low processing temperature of less than 250.degree. C., the ammonia-based chemistry for the formation of silicon oxy-nitride films causes many problems in the quality of the films produced. Problems such as porosity and low density are believed to have been caused by the high content of hydrogen atoms in the film contributed by the hydrogen-rich ammonia. The silicon oxy-nitride films formed at such low processing temperatures by the ammonia-based chemistry contain large numbers of pores which are detrimental to the function of the film as a passivation layer. A porous passivation layer would not longer serve its protective function against either physical abrasion or the penetration of contaminants. The conventional method of using ammonia-based chemistry in producing silicon oxy-nitride films on thin film transistors is therefore unacceptable to the TFT industry.
It is therefore an object of the present invention to provide a method of producing silicon oxy-nitride films at low processing temperatures without using an ammonia-based chemistry.
It is another object of the present invention to provide a method of producing silicon oxy-nitride films on thin film transistors by a PECVD process at a processing temperature lower than 250.degree. C. substantially without using ammonia as a reactant gas.
It is a further object of the present invention to provide an improved method of producing silicon oxy-nitride films on thin film transistor substrates by a PECVD method in which a deposition rate of higher than 200 nm/Min can be achieved at a low processing temperature.